Summary The myocardium possesses an inherent capacity for cellular replacement, yet this reparative process is inadequate to cope with the massive cell death during acute injury or chronic stress. Adoptively transferred stem cell populations showed promise in clinical trials but the efficacy of donated cells to generate new myocardium or lasting gain in myocardial function, remains modest. Extremely low retention and survival of transplanted stem cells and decreased functional activity of autologous stem cells from patients with established disease and co-morbid factors like diabetes may explain limited success with stem cell therapies. It is likely that the molecular signals produced by injured myocardium and extracellular environment are not favorable for stem cell survival, differentiation, migration, and integration. These limitations of stem cell-based therapies warrant alternate strategies to enhance efficiency of cell based therapies. Stem cell-derived- exosomes provide one such alternate cell-free therapeutic modality. Novel, non-traditional use of cell-free components of stem cells such as exosomes, which are loaded with parent stem cell-specific miRs and proteins may allow for harnessing the regenerative power of these cells, without the burden of stem cell viability and differentiation, to augment and modulate endogenous protection and repair processes in the ischemic myocardium. Studies proposed in this PPG therefore put-forth a novel concept and focused and in-depth investigation into the biology of exosome characterization, signaling and function in the context of both small and large animal myocardial repair. Project 1 (Kishore) examines the role of stresses like inflammation and diabetes on the functional properties of exosomes isolated from bone marrow endothelial progenitor cells as well as other stem cells. Project 2 (Walter Koch) focuses upon the involvement of adrenergic receptors and G- protein coupled kinases on cardiac progenitor cell-derived exosomes. Project 3 (Houser) is concerned with Cortical bone stem cell exosome characterization and function. All 3 projects involve in-depth molecular and physiological studies comprising of small and large animal models of myocardial infarction. Establishing alternate sources of stem cell based therapies, such as exosomes, may overcome the impediments to direct cellular replacement leading to functional myocardium and improved hemodynamic performance. Concurrent enhancement therapies to potentiate healing can then benefit from improved endogenous functional repair, leading to more effective compensation of the heart to pathologic stress. Projects in this program will demonstrate exosomes as the significant mediator of both stem cell function and dysfunction, molecular mechanisms responsible for loss of reparative capacity of exosomes and means to improve their functional capacity by directly modifying identified molecules such as proteins and specific microRNAs that create non- permissive conditions for efficient myocardial repair. The goal of this program will be to delineate exosome mediated signaling mechanisms and determine how they can be utilized to restore and enhance endogenous cellular repair processes that heal the damaged heart.